miRNAs regulate protein synthesis by suppressing translation or destabilizing mRNAs. Their role in development and function of the nervous system is not well understood. This year, we identified a miRNA that regulates the development and function of synapses in the hippocampus. We have also determined the target mRNA that is regulated by this miRNA and demonstrated that the expression level of the target mRNA is inversely correlated with the identified miRNA. In addition, using overexpression and knockdown approaches, we show that the regulation of the miRNA on synapse development is mediated by its target mRNA. These findings indicate that miRNAs play important roles in the development of synapses.